The present invention relates to vertical synchronization circuits used in vertical deflection circuit systems for cathode-ray tubes and more particularly to a vertical synchronization circuit capable of stably generating pulses synchronized with the vertical synchronizing signal.
A vertical deflection circuit system known heretofore for a cathode-ray tube has comprised a circuit for separating the vertical synchronizing signal from the composite video signal from a video detection circuit, an oscillation circuit for oscillating in synchronism with the vertical synchronizing signal thus separated, and a circuit for forming a saw-tooth wave for vertical deflection in synchronism with the resulting oscillation output signal. Thus, in this known circuit system, a signal for vertical deflection is obtained by using an oscillation circuit which oscillates in synchronism with the vertical synchronizing signal. For this reason, in the case where the vertical synchronizing signal is not introduced as input for a short period for some reason, or in the case where noise becomes admixed, the vertical synchronization is disrupted, or it becomes unstable, whereby full interlacing of the cathode-ray tube becomes impossible. Another problem which arises is that it is necessary to adjust the oscillation frequency of the above mentioned oscillation circuit.
Accordingly, in order to overcome these difficulties, a vertical synchronization circuit of a count-down system, in which the present invention can be applied, has heretofore been proposed. However, in this system, also, with a conventional circuit organization, disruption of the vertical synchronization disadvantageously occurs when the vertical synchronizing pulses are not generated.
Furthermore, there are instances wherein the level of the tip of the vertical synchronizing signal in the composite video signal is constricted relative to the level of the tip of the horizontal synchronizing signal. This is observed in the case where there is a ghost image or in signals which have been transmitted via a large number of satellite stations. In such a case, the vertical synchronization becomes unstable. In addition, with a conventional circuit organization, in the case where the pulse width of the separated input vertical synchronizing signal is very narrow in the pulse forming circuit, or in the case where noise is admixed immediately before the input vertical synchronizing signal, erroneous operation occurs, whereby formation of normal vertical synchronizing pulses cannot be carried out.